This invention relates to weaving looms which are provided with at least two devices by means of which a weft thread is inserted into the shed from one side to the other side, these devices being disposed one above the other at different levels on the weaving loom, so that weft threads can be inserted simultaneously at these different levels into sheds formed above one another, as is the case, for example, with face-to-face weaving looms provided with two devices which can insert a weft thread simultaneously into the shed of top fabric and bottom fabric.
The invention relates more particularly to weaving looms in which the abovementioned devices are gripper devices.
In the case of known weaving looms with two or more gripper devices, each gripper device consists of two interacting parts which are disposed on either side of the weaving loom, each comprising a drive device with a drive gear disposed on the drive shaft, a gripper bar extending in the breadthwise direction of the weaving loom, which bar can be driven by the drive gear so that it moves with one end from the side of the weaving loom along a guide device to the center and back, while a gripper is fixed on said end of the gripper bar.
The two parts of a gripper device interact in such a way that the grippers meet each other in the center of the weaving loom (in the shed). At that point a weft thread carried from one side of the weaving loom by one gripper (the giver gripper) is taken over by the other gripper (the taker gripper), which then takes this weft thread along on its return movement to the other side of the weaving loom.
During single-spool face-to-face weaving, one weft is inserted alternately into the top fabric and into the bottom fabric.
For such a weaving process, use is made of known single-gripper weaving looms. These looms are provided with a single weft insertion device, for example, a gripper device of the type described above, which in each case inserts the weft thread at the same level. These looms are also provided with weaving frames which pull the foundation and pile warp threads towards the level of the gripper, forming a shed. In this weaving method the warp threads must thus be pulled in such a way that the shed is formed between warp threads extending above the level of the gripper and warp threads extending below that level, the gripper being disposed at a fixed level or moving along with the sley at a fixed level. In the beating-up position of the reed, all foundation warp threads of the top web must be in their top position, and all foundation warp threads of the bottom web must be in their bottom position, in such a way that these foundation warp threads develop a great enough force component to pull the pile threads, in order to obtain sufficient pile height between bottom fabric and top fabric in this way.
The disadvantage of single-spool face-to-face weaving with such a single-gripper weaving loom lies in the fact that the foundation warp threads have to move quickly in order to obtain sufficient lift in a sufficiently short time. In particular, the foundation warp threads of the top fabric have to carry out a fairly great lift during a short period of time or, in other words, during a small angular displacement of the main shaft of the weaving loom.
European Patent Application No. 0,362,433 describes a device which means a possible solution to the abovementioned disadvantage.
This device comprises a weft insertion device which moves up and down vertically and is driven so that it inserts a weft thread successively at different levels. This makes it possible to weave a face-to-face fabric with a single spool, by making the foundation warp threads form a shed at two different levels above one another, as is usual in the case of a double-gripper weaving loom. The advantage lies in the fact that the foundation warp threads have to undergo less lift, and that the foundation warp frames can consequently be moved at a much lower speed, which has major advantages in terms of wear and energy consumption of the machine.
However, this device also has major disadvantages.
A first disadvantage lies in the fact that considerable means are necessary to move a gripper device--which consists of a drive, a gripper guide and a gripper on a gripper bar--constantly up and down, so that this is a time-consuming and expensive solution to the abovementioned disadvantage of a single-gripper weaving loom.
A second disadvantage of the above-described device lies in the fact that the device for moving the weft insertion device up and down will be highly susceptible to wear.
Another known solution to the problem is to weave the fabrics which are to be woven with a single spool on a double-gripper weaving loom. Such a weaving loom is provided with two gripper devices which lie one above the other, and which are provided in order to insert a weft in a shed in the top fabric and a weft in a shed in the bottom fabric simultaneously during double-spool weaving.
Providing such a weaving loom with an additional device, which alternately gives only a weft thread to, for example, the top gripper device and in a subsequent pick only a weft thread to the bottom gripper device, means that this double-gripper weaving loom can be used to weave with a single spool, so that the advantage is obtained that the foundation warp threads need only undergo a small lift, and consequently the foundation warp frames have a long enough movement time, and the movement speeds required can thus be lower.
However, this device also has a disadvantage. During each shot cycle (pick), the drive devices of both weft insertion devices drive their respective grippers to the center of the weaving loom and into the shed, so that there is always one gripper device which is operating needlessly. The energy used for driving this gripperdevice therefore has no beneficial effect at all, and is thus completely lost.
The object of the present invention is then to provide a double-gripper weaving loom by means of which it is possible to weave with a single spool, while the abovementioned disadvantage--the needless driving of a gripper device--is eliminated.
Another object is to provide for a weaving loom with more than two-gripper devices, with which it is possible without the same disadvantage--the needless operation of at least one gripper device --to weave certain weaves which do not require all gripper devices to be driven simultaneously.